Backwashing of Greensand Filter


Oliver Zhae

              Backwashing should occur when the head loss reaches about 69 kpa (10 psi.) and the duration of the backwash should be around 10 to 15 minutes allowing the system to unclog the settled insoluble iron and manganese oxides trapped in the filter. Filter cracking can occur which will affect apparent head loss. Filters should be backwashed everyday, but no less than every 2 days to prevent cracking. It is very important not to underfeed the amount of permanganate added to the pretreatment process or else the greensand filter will lose its oxidative properties. However, if the potassium permanganate charge is somehow lost in the filter, the operator can regenerate the greensand manually. The filter must be first shut down. Then, a saturated solution of potassium permanganate (around 5%) is poured into the filters and left to sit for 24 hours.
After 24 hours, the system is backwashed and restarted. Another way the system can be recharged without shutting down is by increasing the potassium permanganate dosage until pink water flows out of the bottom of the greensand filter. When the pink water flows out of the filter, the filter is recharged and regular doses of potassium permanganate can continue.
The operator should perform iron, manganese, pH and chlorine residual tests on a daily basis in order to determine if there are any problems in the system. Remember, the above is only meant as a guide. Specific backwash requirement are site and equipment specific. Refer to manufacturer specification and procedures as they relate to your plant.

For your safety
When mixing, always add chemicals to water. Never add water to chemicals

The recommend backwash rate for manganese greensand is 12 gpm/sq. ft. of filter area at 60 degrees Farenheit. This rate is sufficient to expand the bed 35–40 percent. Please note that backwash rates versus filter loading rates can cause serious problems in smaller treatment units. For example, a small installation with one 12-inch inside diameter filter, will require the well pump to deliver 12 gpm to properly backwash. However, if high levels of iron are present, that same unit may only be capable of filtering 2 gpm.


The backwash cycle is used to remove impurities that have collected in the media bed. When the backwash cycle is initiated, the backwash inlet valve must not open instantaneously. With the high flow rates used in the backwash cycle, "water hammer" will occur if the valve is opened quickly. "Water hammer" can disrupt the support layer of the greensand filter. The control system must be able to control the opening speed of this valve to eliminate "water hammer."
During the backwash cycle, the valves are oriented to reverse the flow of water from normal operation. With sufficient flow, impurities are loosened from the media bed and carried out of the bessed through the inlet distributor and service inlet. The media bed must be expanded by 30% for the backwash to be effective. To prevent filter media particles escaping from the vessel, the inlet distributor must be sufficiently higher than the top of the expanded bed. The valve configuration used during backwash cycle of the greensand filter system is shown in below Figure.
  • Service Inlet valve closed (to prevent incoming water from flowing against the backwash flow)
  • Service Outlet valve closed (to prevent dirty washback water from contaminating downstream equipment)
  • Backwash Inlet valve open (to provide a supply of water from the hub/lateral underdrain to backwash the media bed)
  • Backwash Outlet valveopen (to set the flow rate and carry away the dirty backwash water from the inlet distributor to drain)
  • Rinse Outlet valve closed (to prevent water from the wrong part of the vessel going to drain)
Backwash continues for a specified time (usually 15 minutes). After the backwash cycle is complete, the vessel is rinsed and can then return to normal service.

Backwash Flow
The backwash flow rate is equal to the flow rate required to increase the bed depth by 30%. The flow rate depends on temperature, since the force pushing the particles up is function of the viscosity of the water, which decreases with increasing temperature. The sub-surface wash uses the same flow rate as the backwash. The table below shows the flow rate based on temperature.
  Backwash Flow = Backwash Flow Rate x Diameter2 x π/4
Greensand Filter Backwash Rates Table
Temperature (°F)
Flow (gpm/ft2)
32 to < 40
7.2
40 to < 50
8.4
50 to < 60
9.7
60 to < 70
11.0
70 to < 80
12.8
80 to < 90
14.3
90 to < 100
16.0
100 to < 110
17.8
110 to 120
19.6

The greensand media has a fixed amount of iron and manganese it can remove before it gets exhausted and needs to be regenerated. It depends on the amount of iron and manganese you have and if there is hydrogen sulfide (“rotten-egg odor”) present.
0 comments

Industrial Wastewater Treatment


Oliver Zhae

Industrial wastewater treatment
Industrial wastewater treatment covers the mechanisms and processes used to treat waters that have been contaminated in some way by anthropogenic industrial or commercial activities prior to its release into the environment or its re-use.
Most industries produce some wet waste although recent trends in the developed world have been to minimise such production or recycle such waste within the production process. However, many industries remain dependent on processes that produce wastewaters.
Sources of industrial wastewater
Agricultural waste
Main article: Agricultural wastewater treatment
Iron and steel industry
The production of iron from its ores involves powerful reduction reactions in blast furnaces. Cooling waters are inevitably contaminated with products especially ammonia and cyanide. Production of coke from coal in coking plants also requires water cooling and the use of water in by-products separation. Contamination of waste streams includes gasification products such as benzene, naphthalene, anthracene, cyanide, ammonia, phenols, cresols together with a range of more complex organic compounds known collectively as polycyclic aromatic hydrocarbons (PAH).
The conversion of iron or steel into sheet, wire or rods requires hot and cold mechanical transformation stages frequently employing water as a lubricant and coolant. Contaminants include hydraulic oils, tallow and particulate solids. Final treatment of iron and steel products before onward sale into manufacturing includes pickling in strong mineral acid to remove rust and prepare the surface for tin or chromium plating or for other surface treatments such as galvanisation or painting. The two acids commonly used are hydrochloric acid and sulfuric acid. Wastewaters include acidic rinse waters together with waste acid. Although many plants operate acid recovery plants, (particularly those using Hydrochloric acid), where the mineral acid is boiled away from the iron salts, there remains a large volume of highly acid ferrous sulfate or ferrous chloride to be disposed of. Many steel industry wastewaters are contaminated by hydraulic oil also known as soluble oil.
Mines and quarries
Mine wastewater effluent with neutralized pH from tailing runoff. Taken in Peru.
The principal waste-waters associated with mines and quarries are slurries of rock particles in water. These arise from rainfall washing exposed surfaces and haul roads and also from rock washing and grading processes. Volumes of water can be very high, especially rainfall related arisings on large sites. Some specialized separation operations, such as coal washing to separate coal from native rock using density gradients, can produce wastewater contaminated by fine particulate haematite and surfactants. Oils and hydraulic oils are also common contaminants. Wastewater from metal mines and ore recovery plants are inevitably contaminated by the minerals present in the native rock formations. Following crushing and extraction of the desirable materials, undesirable materials may become contaminated in the wastewater. For metal mines, this can include unwanted metals such as zinc and other materials such as arsenic. Extraction of high value metals such as gold and silver may generate slimes containing very fine particles in where physical removal of contaminants becomes particularly difficult.
Food industry
Wastewater generated from agricultural and food operations has distinctive characteristics that set it apart from common municipal wastewater managed by public or private wastewater treatment plants throughout the world: it is biodegradable and nontoxic, but that has high concentrations of biochemical oxygen demand (BOD) and suspended solids (SS).[1] The constituents of food and agriculture wastewater are often complex to predict due to the differences in BOD and pH in effluents from vegetable, fruit, and meat products and due to the seasonal nature of food processing and postharvesting.
Processing of food from raw materials requires large volumes of high grade water. Vegetable washing generates waters with high loads of particulate matter and some dissolved organics. It may also contain surfactants.
Animal slaughter and processing produces very strong organic waste from body fluids, such as blood, and gut contents. This wastewater is frequently contaminated by significant levels of antibiotics and growth hormones from the animals and by a variety of pesticides used to control external parasites. Insecticide residues in fleeces is a particular problem in treating waters generated in wool processing.
Processing food for sale produces wastes generated from cooking which are often rich in plant organic material and may also contain salt, flavourings, colouring material and acids or alkali. Very significant quantities of oil or fats may also be present.
Complex organic chemicals industry
A range of industries manufacture or use complex organic chemicals. These include pesticides, pharmaceuticals, paints and dyes, petro-chemicals, detergents, plastics, paper pollution, etc. Waste waters can be contaminated by feed-stock materials, by-products, product material in soluble or particulate form, washing and cleaning agents, solvents and added value products such as plasticisers. Treatment facilities that do not need control of their effluent typically opt for a type of aerobic treatment, i.e. Aerated Lagoons.[2]
Nuclear industry
The waste production from the nuclear and radio-chemicals industry is dealt with as Radioactive waste.
Water treatment
Water treatment for the production of drinking water is dealt with elsewhere. (See water purification.) Many industries have a need to treat water to obtain very high quality water for demanding purposes. Water treatment produces organic and mineral sludges from filtration and sedimentation. Ion exchange using natural or synthetic resins removes calcium, magnesium and carbonate ions from water, replacing them with hydrogen and hydroxyl ions. Regeneration of ion exchange columns with strong acids and alkalis produces a wastewater rich in hardness ions which are readily precipitated out, especially when in admixture with other wastewater.
Treatment of industrial wastewater
The various types of contamination of wastewater require a variety of strategies to remove the contamination.[3][4]
Solids removal
Most solids can be removed using simple sedimentation techniques with the solids recovered as slurry or sludge. Very fine solids and solids with densities close to the density of water pose special problems. In such case filtration or ultrafiltration may be required. Although, flocculation may be used, using alum salts or the addition of polyelectrolytes.
Oils and grease removal
Main article: API oil-water separator
A typical API oil-water separator used in many industries
Many oils can be recovered from open water surfaces by skimming devices. Considered a dependable and cheap way to remove oil, grease and other hydrocarbons from water, oil skimmers can sometimes achieve the desired level of water purity. At other times, skimming is also a cost-efficient method to remove most of the oil before using membrane filters and chemical processes. Skimmers will prevent filters from blinding prematurely and keep chemical costs down because there is less oil to process.
Because grease skimming involves higher viscosity hydrocarbons, skimmers must be equipped with heaters powerful enough to keep grease fluid for discharge. If floating grease forms into solid clumps or mats, a spray bar, aerator or mechanical apparatus can be used to facilitate removal.[5]
However, hydraulic oils and the majority of oils that have degraded to any extent will also have a soluble or emulsified component that will require further treatment to eliminate. Dissolving or emulsifying oil using surfactants or solvents usually exacerbates the problem rather than solving it, producing wastewater that is more difficult to treat.
The wastewaters from large-scale industries such as oil refineries, petrochemical plants, chemical plants, and natural gas processing plants commonly contain gross amounts of oil and suspended solids. Those industries use a device known as an API oil-water separator which is designed to separate the oil and suspended solids from their wastewater effluents. The name is derived from the fact that such separators are designed according to standards published by the American Petroleum Institute (API).[4][6]
The API separator is a gravity separation device designed by using Stokes Law to define the rise velocity of oil droplets based on their density and size. The design is based on the specific gravity difference between the oil and the wastewater because that difference is much smaller than the specific gravity difference between the suspended solids and water. The suspended solids settles to the bottom of the separator as a sediment layer, the oil rises to top of the separator and the cleansed wastewater is the middle layer between the oil layer and the solids.[4]
Typically, the oil layer is skimmed off and subsequently re-processed or disposed of, and the bottom sediment layer is removed by a chain and flight scraper (or similar device) and a sludge pump. The water layer is sent to further treatment consisting usually of a Electroflotation module for additional removal of any residual oil and then to some type of biological treatment unit for removal of undesirable dissolved chemical compounds.
A typical parallel plate separator
Parallel plate separators[7] are similar to API separators but they include tilted parallel plate assemblies (also known as parallel packs). The parallel plates provide more surface for suspended oil droplets to coalesce into larger globules. Such separators still depend upon the specific gravity between the suspended oil and the water. However, the parallel plates enhance the degree of oil-water separation. The result is that a parallel plate separator requires significantly less space than a conventional API separator to achieve the same degree of separation.
Removal of biodegradable organics
Biodegradable organic material of plant or animal origin is usually possible to treat using extended conventional wastewater treatment processes such as activated sludge or trickling filter.[3][4] Problems can arise if the wastewater is excessively diluted with washing water or is highly concentrated such as neat blood or milk. The presence of cleaning agents, disinfectants, pesticides, or antibiotics can have detrimental impacts on treatment processes.
Activated sludge process
Main article: Activated sludge
A generalized, diagram of an activated sludge process.
Activated sludge is a biochemical process for treating sewage and industrial wastewater that uses air (or oxygen) and microorganisms to biologically oxidize organic pollutants, producing a waste sludge (or floc) containing the oxidized material. In general, an activated sludge process includes:
  • An aeration tank where air (or oxygen) is injected and thoroughly mixed into the wastewater.
  • A settling tank (usually referred to as a "clarifier" or "settler") to allow the waste sludge to settle. Part of the waste sludge is recycled to the aeration tank and the remaining waste sludge is removed for further treatment and ultimate disposal.
  • As a general process for most of the Industrial waste water the following Technologies are used.
1. ASP : Activated Sludge process 2. SAFF system of Submerged aerobic fixed film system 3. MBBR : Moving bed bio reactor ( Anox invented this now is considered generic technology) 4. MBR : Membrane Bioreactor 5. DAF clarifiers 6. TBR : Turbo bioreactor Technology ( A patented technology of Wockoliver) 7. Filtration technologies More information about above can be found on various commercial manufactures like WOIL
Trickling filter process
Main article: Trickling filter
Image 1: A schematic cross-section of the contact face of the bed media in a trickling filter
A typical complete trickling filter system
A trickling filter consists of a bed of rocks, gravel, slag, peat moss, or plastic media over which wastewater flows downward and contacts a layer (or film) of microbial slime covering the bed media. Aerobic conditions are maintained by forced air flowing through the bed or by natural convection of air. The process involves adsorption of organic compounds in the wastewater by the microbial slime layer, diffusion of air into the slime layer to provide the oxygen required for the biochemical oxidation of the organic compounds. The end products include carbon dioxide gas, water and other products of the oxidation. As the slime layer thickens, it becomes difficult for the air to penetrate the layer and an inner anaerobic layer is formed.
The components of a complete trickling filter system are: fundamental components:
  • A bed of filter medium upon which a layer of microbial slime is promoted and developed.
  • An enclosure or a container which houses the bed of filter medium.
  • A system for distributing the flow of wastewater over the filter medium.
  • A system for removing and disposing of any sludge from the treated effluent.
The treatment of sewage or other wastewater with trickling filters is among the oldest and most well characterized treatment technologies.
A trickling filter is also often called a trickle filter, trickling biofilter, biofilter, biological filter or biological trickling filter.
Treatment of other organics
Synthetic organic materials including solvents, paints, pharmaceuticals, pesticides, coking products and so forth can be very difficult to treat. Treatment methods are often specific to the material being treated. Methods include Advanced Oxidation Processing, distillation, adsorption, vitrification, incineration, chemical immobilisation or landfill disposal. Some materials such as some detergents may be capable of biological degradation and in such cases, a modified form of wastewater treatment can be used.
Treatment of acids and alkalis
Acids and alkalis can usually be neutralised under controlled conditions. Neutralisation frequently produces a precipitate that will require treatment as a solid residue that may also be toxic. In some cases, gasses may be evolved requiring treatment for the gas stream. Some other forms of treatment are usually required following neutralisation.
Waste streams rich in hardness ions as from de-ionisation processes can readily lose the hardness ions in a buildup of precipitated calcium and magnesium salts. This precipitation process can cause severe furring of pipes and can, in extreme cases, cause the blockage of disposal pipes. A 1 metre diameter industrial marine discharge pipe serving a major chemicals complex was blocked by such salts in the 1970s. Treatment is by concentration of de-ionisation waste waters and disposal to landfill or by careful pH management of the released wastewater.
Treatment of toxic materials
Toxic materials including many organic materials, metals (such as zinc, silver, cadmium, thallium, etc.) acids, alkalis, non-metallic elements (such as arsenic or selenium) are generally resistant to biological processes unless very dilute. Metals can often be precipitated out by changing the pH or by treatment with other chemicals. Many, however, are resistant to treatment or mitigation and may require concentration followed by landfilling or recycling. Dissolved organics can be incinerated within the wastewater by Advanced Oxidation Process.

References

  1. ^ European Environment Agency. Copenhagen, Denmark. "Indicator: Biochemical oxygen demand in rivers (2001)."
  2. ^ Tannery Wastewater Treatment by the Oxygen Activated Sludge Process Mamoru Kashiwaya and Kameo Yoshimoto Journal (Water Pollution Control Federation), Vol. 52, No. 5 (May, 1980), pp. 999-1007 (article consists of 9 pages) Published by: Water Environment Federation
  3. ^ a b Tchobanoglous, G., Burton, F.L., and Stensel, H.D. (2003). Wastewater Engineering (Treatment Disposal Reuse) / Metcalf & Eddy, Inc. (4th ed.). McGraw-Hill Book Company. ISBN 0-07-041878-0.
  4. ^ a b c d Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st ed.). John Wiley & Sons. LCCN 67019834.
  5. ^ Water and Wastewater News, May 2004 <http://wwn-online.com/articles/50898/>
  6. ^ American Petroleum Institute (API) (February 1990). Management of Water Discharges: Design and Operations of Oil-Water Separators (1st ed.). American Petroleum Institute.
  7. ^ a b Beychok, Milton R. (December 1971). "Wastewater treatment". Hydrocarbon Processing: 109–112. ISSN 0818-8190.


4 comments

Activation of Raw Water Pre-Treatment


Oliver Zhae



JURNAL TEKNOLOGI, Edisi No.4 Tahun XX, Desember 2006, 287-291 ISSN 0215-1685





Activation of Raw Water Pre-Treatment Facility in PLTU Ombilin


Misri Gozan, Emil Mahfuzi, Lili Hambali dan Asep Handaya Saputra
Chemical Engineering Department, Faculty of Technology, University of Indonesia
Kampus Universitas Indonesia, Depok 16424, Indonesia
e-mail: mgozan@che.ui.edu

Abstrak
Pada sebuah Pembangkit Listrik Tenaga Uap (PLTU), seperti di PLTU Ombilin, Sumatera Barat, air
digunakan  terutama untuk air  umpan boiler,  untuk pendingin, pemadam  kebakaran, service  water, dan
air  minum.  PLTU  Ombilin  menggunakan  Sungai  Ombilin  sebagai  sumber  air  baku  untuk  memenuhi
semua kebutuhannya akan air. Sebelum dapat digunakan untuk memenuhi semua keperluan tersebut, air
baku  harus  diolah  terlebih  dahulu  melalui  berbagai  tahapan  untuk  menghilangkan  berbagai  pengotor
yang secara alami terkandung di dalamnya. Penelitian ini bertujuan untuk merancang unit pengolahan
awal  air  baku  untuk  utilitas  di  PLTU  Ombilin  dan  membandingkannya  dengan  unit  pengolahan  yang
sudah ada dan beroperasi. Untuk keperluan perancangan digunakan data laju alir air yang diolah yang
diperoleh  dari  lapangan  yaitu  sebesar  1.160  m3/jam.  Perancangan  ditujukan  untuk  menyisihkan  TSS,
patogen,  dan  kekeruhan  yaitu  dengan  menggunakan  prinsip-prinsip  koagulasi,  flokulasi,  sedimentasi,
disinfeksi, dan membran ultrafiltrasi. Hasil perancangan ini berupa rangkaian proses yang tersusun dari
static mixer, flokulator, clarifier, dan membran ultrafiltrasi, serta dengan menggunakan senyawa kimia
meliputi  alum,  kapur,  NaOCl,  dan  polielektrolit.  Static  mixer  yang  digunakan  memiliki  diameter  pipa
sebesar  16  in.  Flokulator  dirancang  berupa  saluran  berpenampang  (2x2)  m2  dengan  panjang  100  m.
Clarifier berupa unit aliran horizontal, dengan permukaan (40 x 20) m2 dan kedalaman 5,8 m. Clarifier
ini memiliki laju beban permukaan 35 m3/m2.d, laju beban weir 250 m3/m.d  dengan panjang weir 111,36
m.  Membran  ultrafiltrasi  hanya  mengolah  80%  air  umpan,  dengan  fluks  50  lmh,  dan  luas  permukaan
yang dibutuhkan 13.290 m2. Dari perbandingan hasil perancangan dan unit pengolahan yang sudah ada,
didapatkan rekomendasi bagi unit pengolahan yang ada untuk tidak menggunakan screen, memodifikasi
flokulator dan clarifier, serta mengganti saringan pasir dengan membran ultrafiltrasi.
Kata kunci: Pengolahan air, filtrasi, sedimentasi, koagulasi dan flokulasi umpan boiler.

Abstract
In a power supply (PLTU) located in West Sumatera Barat, water is massively utilized for boiler feed,
chiller fire fighters, service water, and drinking water. This need is supplied by a river nearby as the only
source. This raw water   is subject to a pre-treatment unit to remove the contaminants. This research was
aimed to design a raw water pre treatment unit for utility in the power supply and to compare with the
existing and operating treatment unit. The feed flow rate is 1,160 m3/hour. The design objective was to
remove   TSS,   pathogenic   bacteria,   and   turbidity   using   coagulation,                                 flocculation,   sedimentation,
disinfectant, and ultra filtration membrane. Static mixer was used with pipe diameter 16 in. Flocculator
has  tubular  size  of  (2x2)  m2  with  100  m  length.  Clarifier  was  a  horizontal  flow  type  with  surface  of
(40x20) m2 and depth 5,8 m. This clarifier has surface loading flow rate of  35 m3/m2.d, weir loading 250
m3/m.d        with weir length of 111.4 m. Ultra-filtration membrane treats only 80% of feed water (50 lmh
flux) and need surface area of 13,290 m2. From the comparison we found that we do not the screening.
Furthermore,  we  should  modify  the  flocculator  and  clarifier,  and  replace  the  sand  filtration  unit  with
ultra-filtration membrane.
Keywords: Water treatment, filtration, sedimentation, coagulation, flocculation and boiler feed.


1. Introduction


A   power   supply   in  Sawahlunto,  West Sumatera  takes  water  from  Ombilin  river and  treats  the  water  for  several  purposes. Water    fed    to    boiler    should    be    free    of
mineral.   Therefore,   raw   water   undergoes pre      treatments      and      finally      goes      to





Parameter
Nilai
Total Suspended Solid
< 1 mg/l
Turbidity
< 0,5 JTU
Free chlorine
0,1 – 0,5 mg/l

 


Parameter
value
Total Suspended Solid
< 5 mg/l
Turbidity
< 2 JTU
Free chlorine
0,1 – 1 mg/l

 

M. Gozan, E. Mahfuzi, L. Hambali dan A. H. Saputra





demineralisation  plant.  Water  goes  through
belt       screen,        static        mixer,        floculator,
clarifier,  and  sand  filter.  Petreatment  plays
an  important  role  prior  to  demineralisation
to     remove        most      of     the      organic        and
inorganic.
In   the   pretreatment   stage,   Aluminum
sulphate         (Al2(SO4)3),         lime        (Ca(OH)2),
NaOCl,  and  a  poly-electrolite  are  used  to
remove   the   suspended   solid.   Function   of

Alum  as  coagulant,  lime  as  pH  regulator,
NaOCl  as  disinfektan,  and  polielektrolit  as
(coagulant  aid)  or  flokulan.  To  re-activate
raw water pre-treatment   need a study again
to the existing unit

2. Metodology

The   objectives   of   this   research   is   to
obtain      first     design      stage      of    raw     Water
pretreatment            for   Ombilin   Steam   Power
Plant  for  utility  by  several  process  stage,
include  rapid  mixing,  low  mixing  and  ultra
filtration   membrane.   The   equipments   are
designed   for   rapid   mixing,   sedimentation
tank and ultra filtration for fine strain.
Raw      water       has      strict      requirements
criteria,  even  more  strict  than  requirement
of domestic water. Boiler fed water must be
free   of   mineral   ingredients   in   raw   water,
therefore   fed   water   must   be   proceed   in
deminerasation              to         remove           mineral
ingredients.  Demineralisation  can  be  run  in
several       methods,        one     of     them      is     ion
exchange. The design is performed base on
datas  which  has  taken  on  site  (the  existing
water pretreatment).others, asumption datas
are used in calculation.

3. Result and discussion

Water         pre-treatmentis             located          in
Figure 1.
Pretreatment Process Schematics

The  raw  water  from  river  is  add  with
chemical    coumpound    which    consist    of
alum,   lime,   natrium   hipoklorit   (NaOCl).
The coumpound is feed to a static mixer.
Raw     Water     pretreatment     in     Steam
power    plant    Ombilin    produce    water    to
fulfill   need   of   demineralisation   and   other
neccesities    such    as    service    water,    fire
fighting  water  and  cooling  water.  Most  of
ouput water of Clarifier (80%) need further
treatment by using sand filter, others is used
for system service, fire fighting and cooling
water   system.   Sand   filter   produce   water
where   fulfil   requirement   water   feed   for
demineralisation.   Quality   of   water   output
clarifier  and  output  of  sand  filter  are  listed
in Table 1.

Tabel 1.
Quality of Water Output Clarifier







Tabel 2.
Quality of Water Output of Sand Filter

Ombilin   Steam   Power   Plant   which   used
principle           of        coagulation,            floculatin,
sedimentation,   disinfectan,   and   filtration.
Equipment         used      include        static       mixer,
screen,   pulsator   clarifier   and   sand   filter.
Schematic  process  of  water  pretreament  is
shown in Figure 1




288                                                   JURNAL TEKNOLOGI, Edisi No.4 Tahun XX, Desember 2006, 287-291





No.
Equipment
Spesification
Value
1
Static Mixer
Flow (Q)
1.160 m3/h


Pipe Diameter (D)
16 in


Velocity (v)
0,621 m/s


Reynold     Number
(R)
3,02 x 106
2
Flocculator
Flow(Q)
1.160 m3/h


Cross section (A)
4 m2


Eometry   of   cross
section G
(2 x 2) m2


detenti on time(t)
20 minutes


Volume (V)
386,4 m3


Channel length (l)
100 m
3
Clarifier
Flow (Q)
1.160 m3/h


Surface            load
velocity
35 m3/m2.d


weir load velocity
250 m3/m.d


Surface area (A)
795,4 m2


Surface shape
(40    x  20)
m2


High (H)/ Depth
5,8 m


Weir length
111,36 m
4
Ultra
filtration
membrane
feed Flow (Q)
928 m3/day

permeate flow
696 m3/day


permeate flux(J)
50         lmh
(litre       per
m2         per
hour)


Surface area (A)
13.920 m2

 
Activation of Raw Water Pre-Treatment Facility in PLTU Ombilin






The design is summarized on table 3.

Table 3.
Summary of Design Result






































There   are   several   differences   between
design       result      with      existing       plant,       the
differences are the usage of screen, clarifier
design,  and  the  difference  of  the  usage  of
sand filter and ultra filtration membrane.
Flocculator channel
The      result       of      design       shown       that
flocculator channel 100 meter in length and
(2   x   2)   m2   cross   section.   Meanwhile   on
site,  flocculator  channel  about  30  meter  in




length  with  (1  x  1)  m2  in  cross  section.  It
cause   imperfection   in   floculation   because
detention is not sufficient
To improve performance of flocculation
of operated water treatment units, floculator
channel   should   be   modified.   It   could   be
modified   as   calculation   result   change,   so
that  floc  can  be  obtained  and  easy  to  form
sediment.
Clarifier Design
Clarifier      design      as      a      result      of
calculation  differ  with  existing  clarifier  on
site. It is occured because basic operation is
different.     The     design     of     clarifier     use
horizontal     flow     meanwhile     exist     plant
special   type   of   flow   is   clarifier   pulsator
vertical flow.
Pulsator  system  use  fan  to  vacuum  the
vacuum  chamber  on  surface  of  water.  As
fan  cause  chamber  vacuum,  the  surface  of
water would flow up in vertical direction in
the   chamber.   a   moment,   i.e.   40   seconds,
then  the  vacuum  would  be  released  so  that
the      surface      of      water      would      down
immediately.     20     seconds     then     vacum
chamber  would  be  vacuum  again  and  the
process  will  be  done  countinuosly  base  on
certain   time   periods.   It   is  combined   with
spread  jet  channel  that  exist  in  clarifier  to
form sludge as a result of pulsation system,
they  spread  on  the  clarifier.  By  using  this
method,   sedimentation   process   would   be
faster  because  flocs  would  not  only  form
sediment   because   of   gravitation   but   also
force on pulsation process.

4. Summary

Water Pre-treatment Plant for the utility
use      coagulant      principle,      flocculation,
sedimentation,         and         ultra         filtration
membrane need following equipments:
Mixer     Static     is     to     mix     chemical
compound, include alum as coagulant, lime
as pH regulator, and NaOCl as disinfectant.
The  operated  Water  pretreatment  plant
in PLTU Ombilin needs some modification.




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M. Gozan, E. Mahfuzi, L. Hambali dan A. H. Saputra






Floculator        channel       need      to     modify       to
produce sufficient detention time in order to
form  better  flocs.  Pulsator  clarifier  should
be  replaced  with  horizontal  flow  clarifier.
Sand   filter   should   be   replaced   with   ultra
filtration  where  produce  water  with  better
quality.

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Activation of Raw Water Pre-Treatment Facility in PLTU Ombilin





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